It is well known in the art of internal combustion engines to provide for recirculation of a portion of the engine exhaust back into the engine intake to reduce combustion temperatures, reduce formation of unwanted deposits, reduce formation of NOx and SOx compounds, and improve fuel economy. Typically, a pintle valve is disposed between ports in the exhaust manifold and the intake manifold and is actuated by a linear solenoid actuator. In a typical application, the energized solenoid displaces the pintle shaft in either a valve-opening or valve-closing motion, and the reverse motion is carried out by a spring when the solenoid is de-energized.
In some known applications, it is desirable to control the motion of the pintle shaft in both directions. This may be conveniently achieved by driving the pintle shaft with a rotary cam driven by a motor, for example, a stepper or other discrete-motion motor. Correct operation of the valve requires that the cam be rigidly attached to the motor shaft in a position that cancels out the position and orientation assembly tolerance stackup.
The cam position is critical for a number of reasons. The motor and sensor package has only 90° of nominal usable rotation, and a portion of that arc is taken up by other apparatus so that the actual usable rotation is less than 90°. The cam profile must use as much as possible of the actual rotation available in order to translate the rotational motion to linear motion with appropriate force characteristics.
In the prior art, a single laser weld is used to attach the cam to the motor shaft after the cam position and orientation is established with reference to a datum on the assembly housing. This attachment meets an accepted standard for vibrational robustness when tested against a standard vibration schedule. However, this attachment can fail when subjected to a vibration schedule for severe duty, as may be experienced in use on tractors and similar machines.
Analysis of severe-duty failures shows that the weld fails because of fatigue in the heat affected zone on the motor shaft near the fusion zone. FEA analysis shows the stress due to vibrational effects is too high for the joint geometry and therefore supports the empirical failure data.
What is needed in the art is an improved method and apparatus for inexpensively fixing a cam onto a rotary shaft such that the cam remains fixed to the shaft during severe duty.
It is a principal object of the present invention to attach a cam to a rotary shaft in such a way that a) position and orientation tolerance stackup is cancelled, and b) the cam remains fixed to the shaft during severe duty.